The invention relates to a method and a device for monitoring the brake of a motion-monitored and controlled printing press drive motor.
Because of safety regulations rotary machines such as offset printing machines must have their drive trains equipped with a switchable motor brake. This serves on the one hand as a parking or locking brake to safeguard the operator against accidental starting of the machine, and on the other hand as an emergency brake which enables the machine drive to be decelerated and stopped as quickly as possible in an emergency.
Such motor brakes consist normally of brake discs lined with friction material which in a non-powered condition are pressed together by springs. The required brake force is generated by the friction between the two brake discs. When a voltage is applied to the brake the brake discs are axially moved apart against the force of the compression springs thus releasing the brake force.
For function-checking this type of motor brake, limit switches are provided which sense the position of at least one of the movable brake discs. The starting of the drive motor is, for instance, prevented if the brake discs do not move apart after the voltage is applied and thus do not actuate the limit switch or switches which then cannot signal the release of the motor brake.
The disadvantage of such a brake checking system lies in the fact that only the actual position of the brake discs in relation to one another can be monitored; any information on the function of the motor brake and the braking force produced by it is not available because the limit switch or switches are not capable of detecting a reduction of the friction lining thickness produced by normal wear and tear, or any other defects such as contamination of the linings by oil. Apart from that, a wrong positioning of the limit switch or switches or an alteration of the switching points after they have been in operation some time, cannot be entirely excluded, and therefore a certain lack of operational safety is to be expected.